The present invention relates generally to optical systems, and more specifically the invention pertains to a Fabry-Perot interferometer that is shear and tilt immune as well as being tunable.
Optical interferometers are devices in which interference of light is used. Some applications of this phenomenon in which interferometers are used as a tool include metrology and spectroscopy. These uses include precise measurements of wavelength, the measurement of very small distances and thicknesses by using known wavelengths, the detailed study of the hyperfine structure of spectrum lines, the precise determination of refractive indices, and, in astronomy, the measurement of binary-star separations and the diameters of stars. Optical interferometers are based on both two-beam interference and multiple-beam interference, as discussed briefly below.
The interference of waves is the process whereby two or more waves of the same frequency or wavelength combine to form a wave whose amplitude is the sum of the amplitudes of the interfering waves. The interfering waves can be electromagnetic, acoustic, or water waves, or in fact any periodic disturbance.
The most striking feature of interference is the effect of adding two waves in which the trough of one wave coincides with the peak of another. If the two waves are of equal amplitude, they can cancel each other out so that the resulting amplitude is zero. This is perhaps most dramatic in sound waves; it is possible to generate acoustic waves to arrive at a person's ear so as to cancel out noise that is disturbing him. In optics, this cancellation can occur for particular wavelengths in a situation where white light is a source. The resulting light will appear colored. This gives rise to the irridescent colors of beetles' wings and mother-of-pearl, where the substances involved are actually colorless or transparent.
A conventional Fabry-Perot interferometer promotes multiple interference using: focusing optics, two glass plates, two partially transmitting silver layers, an image plane, and a frame upon which the other elements are fixed. The focusing optics often consist of two lens elements: one lens in front of the glass plates, and one lens behind the glass plates. The first lens is a collimating lens to collimate the light entering the interferometer. The glass plates are fixed parallel to each other on the frame and have the partially transmitting silver layers fixed on their interior sides so that the incoming wave is multiply reflected between the two surfaces. The second lens will focus the outgoing wave onto a focal plane.
The conventional Fabry-Perot interferometer functions well as long as the frame holds the optical elements in alignment. If the glass plates and partially transmitting silver layers become tilted with respect to each other, the interferometer will lose its ability to function.
The task of providing a Fabry-Perot interferometer which is immune to the effects of tilt and shear movement is alleviated, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,895,428 issued to Nelson et al; PA0 U.S. Pat. No. 4,466,699 issued to Droesler et al; PA0 U.S. Pat. No. 4,559,224 issued to Monchalin; PA0 U.S. Pat. No. 4,733,561 issued to Gilby; PA0 U.S. Pat. No. 4,768,182 issued to Hatfield; PA0 U.S. Pat. No. 4,859,060 issued to Katagiri et al; PA0 U.S. Pat. No. 4,825,262 issued to Mallinson; PA0 U.S. Pat. No. 4,547,801 issued to Haisma et al; PA0 U.S. Pat. No. 4,542,988 issued to Kraushaar; PA0 U.S. Pat. No. 4,243,323 issued to Breckinridge; and PA0 U.S. Pat. No. 4,225,236 issued to Sandercock.
The patents identified above relate to variable interferometric devices. In particular, the Katagiri et al patent describes a Fabry-Perot interferometer comprising a pair of reflecting substances facing each other with a space in between. A means for deforming at least one of the reflecting substances is provided to change the interferometric characteristics of the device. The means for deforming the reflecting substance may be an electrostatic force or a magnetic field.
The Mallison patent relates to a Fabry-Perot interferometer in which one of the reflective surfaces is affixed on a diaphragm mounted by a hinge assembly to a support. This approach allows the position of the diaphragm to be changed, resulting in the size of the gap to be changed. A single crystal may be used for the support, diaphragm, and hinge assembly.
The Haisma et al patent describes a tunable Fabry-Perot interferometer comprising two parallel facing mirrors where the supports for the mirrors consist of bundles of optical fibers. The mirrors are situated at a very small distance from each other and extend at right angles to the axis of the interferometer. Such a tunable Fabry-Perot interferometer can be used effectively in an x-ray display device.
The Kraushaar patent relates to a reflective Fabry-Perot interferometer where the conventional pair of spaced parallel mirrors have a predetermined angle with respect to an incident light beam. Third and fourth mirrors are each disposed at a different end of the first and second mirrors at a second predetermined angle with respect to a line parallel to the incident light beam. With this approach, end losses in the two parallel conventional mirrors are reduced, and the light beam emanating from the two conventional mirrors is reinforced.
The Breckinridge patent describes an interferometer which is tilt compensated and facilitates adjustment of the path lengths of split light beams. The interferometer comprises a pair of plate-like elements with a dielectric coating and an oil film between them that forms a beamsplitter interface. A pair of reflector surfaces are affixed at the ends of the plates. A pair of retroreflectors are positioned so that each split beam component is directed by a retroreflector onto one of the reflector surfaces and is then returned to the beamsplitter interface. The oil film which is less than twenty micro-inches thick, passes approximately equal amounts of all wavelengths longer than the film thickness, and enables one plate to shift relative to the other.
The Sandercock patent relates to a vernier tandem Fabry-Perot interferometer comprising a first pair of plane mirrors forming an interferometer, and a second pair of plane mirrors forming a second interferometer. One mirror of each pair is mounted on a common movable support so that when the support is moved, the spacing between one pair of mirrors remains in the same ratio to the spacing between the other pair of mirrors as the original ratio between these spacings. The system thus continues to exhibit a single pass band at frequencies of interest corresponding to different spacings between mirror pairs. Mirror parallelism is maintained by a deformable parallelogram, and a transducer provides mirror motions.
All of the above-cited references (except Nelson) describe the technology associated with Fabry-Perot interferometer applications. Unfortunately, the optical elements of such systems remain subject to potential disruption from any tilt and/or shear displacement. The present invention is intended to provide a Fabry-Perot configuration immune to the effects of tilt and shear displacement with the use of phase conjugate or psuedo-phase conjugate mirror technology.